The multiplication of animal cell in vitro, requires the presence of macromolecular growth factors in the culture medium. Serum has normally been the source of these multiplication-stimulating factors. Due to the complexities of serum, the mechanism by which these factors stimulate the growth of cells remains obscure. In these proposed studies, multiplication-stimulating factors will be purified from a unique alternate source, namely conditioned medium. Several cell lines are available which grow in the absence of serum and produce growth-promoting substances and condition the medium. Purified growth factors will then be used to investigate the mechanism by which they stimulate division in serum-dependent cells. Early biochemical changes after the addition of growth factors will be studied and several parameters of membrane function will be monitored. Responses of normal cells, virus-transformed cells and revertants of transformed cells, to purified multiplication-stimulating factors will be compared in attempts to find a biochemical basis for the more efficient utilization of serum factors and the growth advantage of transformed cells. Purified growth factors will also be assayed for non-suppressible insulin-like activity (NSILA) to determine if they are similar to the somatomedins, a family of growth-hormone dependent polypeptides which may be involved in growth control in vitro.